Methyl group dynamics in α-crystallized toluene as studied by deuteron spin–lattice relaxation

Abstract

Deuteron spin–lattice relaxation experiments on α-crystallized toluene-d3 are reported. It is shown that the two distinct methyl groups existing in this crystal can be identified by 2H–T1 experiments. The temperature dependence of the broadenings is found to be different, particularly in an intermediate temperature range, where transitions to the second excited librational state dominate the dynamics for one type of methyl group. For the second type, the crossover from a low temperature behavior to a ‘‘quasiclassical’’ regime takes place in a very narrow temperature range. The apparent activation energies are compared to the known transition energies of protonated toluene.

Deuteron NMR of Methyl Groups in theTunneling Regime. A Single Crystal Study of Aspirin-CD3

“…This conclusion is strengthened by the value of E'a which we obtained from the relaxation data. Theorists think that E'a should be identified with the height of the potential above the ground state level which is, for a purely threefold potential, equal to V3 -E (0) [15]. For V3 = 47.2 meV the solution of the Mathieu equa tion gives E {0) = 5.7 meV, which leads for E'a to the prediction V3 -E (0) = 41.5 meV.…”

“…These sites are, moreover, rather ill-defined [13]. We also felt that the classical systems where, in powder samples, methyl tunneling has been observed previously, namely cop per acetate monohydrate [14] and toluene [15], [16], are anything but simple.…”

“…The increase of T\ on the low temperature side starts to level off at about the same temperature where E-lines are first observed in the spectrum (« 28 K). The departure of log T\ from linearity can be explained by assuming that it is governed by an Arrhe nius law behaviour with different apparent activation energies in different temperature regimes [15]. In the limit T -* 0, it is related to the energy of the first excited librational level, while at higher temperatures transitions to higher librational levels must be taken into account.…”

Methyl and t-butyl group reorientation in planar aromatic solids: Low-frequency nuclear magnetic resonance relaxometry and x-ray diffraction

“…Proton relaxation rate studies, on the other hand, have the advantage that the hydrogen atoms under study can interact strongly with nearby hydrogen atoms ͑spin-spin interactions͒ and this gives a window on relating the observed relaxation rates to both the local and the longer-range structures of the solid. Deuteron NMR spectroscopy and relaxation rate studies 23 and x-ray diffraction 24 have been combined to show that there are two chemically inequivalent methyl group sites in alpha-crystallized toluene. A truly beautiful, more recent deuteron spectroscopy and relaxation rate study in 2,3-dimethylnaphthalene 25 exploited the known crystal structure 26 to relate the disordered structure of this solid with models for methyl group reorientation.…”

Methyl and t‐Butyl Group Reorientation in Planar Aromatic Solids: Low‐Frequency Nuclear Magnetic Resonance Relaxometry and X‐Ray Diffraction

“…Proton relaxation rate studies, on the other hand, have the advantage that the hydrogen atoms under study can interact strongly with nearby hydrogen atoms ͑spin-spin interactions͒ and this gives a window on relating the observed relaxation rates to both the local and the longer-range structures of the solid. Deuteron NMR spectroscopy and relaxation rate studies 23 and x-ray diffraction 24 have been combined to show that there are two chemically inequivalent methyl group sites in alpha-crystallized toluene.…”